Experimental research on the influence of laser-induced plasma on beam focusing during high-power CO2 laser materials processing

摘要

Abstract: During high power CO$-2$/ laser materials processing, the process will be disturbed or interrupted when the propagation plasma generates and detaches from the workpiece. This phenomenon is called the shielding of plasma to the incident laser beam. It is usually considered that the plasma absorption contributes to the plasma shielding. In fact, the plasma just absorbs only about 50% incident laser energy when plasma shielding occurs. Therefore, several theoretical modes have been presented concerning on the plasma refraction and defocusing effect. However, the results presented are not verified by experiments. In this paper, we develop a simple but effective experimental method to verify the effects of plasma on the focused beam propagation according to the basic fact that plasma sustains for a while even when laser beam moves out of the workpiece at high power. It is found that the focus spot enlarges and the position of the focus shifts due to the effects of the plasma. This `Lensing Effect' of the laser induced plasma becomes more serious with the increase of the distance of plasma propagation. We conclude that the `Lensing Effect' of plasma, other than plasma absorption, is the main mechanism for plasma shielding. Due to the `Lensing Effect', the power density on the workpiece decreases seriously which causes the process be interrupted.!27